Reliability and Safety Therac-25

Case Study - Therac-25 Software controlled radiation therapy machine used to treat people with cancer Problems: Massive overdoses administered Repeated overdoses due to faulty display Death of patients Operated in dual machine mode - electron beam or x-ray photon beam

Why Did Problems Arise? Lapses in good safety design Insufficient testing Bugs in software that controlled machines Inadequate system of reporting and investigating accidents and deaths

Specific Problems Some hardware safety features were eliminated in newer models Software used was assumed correct form older systems Malfunctioned frequently requiring operator intervention Weakness in design of operator interface Inadequate explanation of error messages if any

Specific Problems continued… Machine allowed one-key intervention versus automatic shutdown Inadequate documentation Poor test plan

Software Errors - Bugs Fatal error was a simple fix Fixes are complex, expensive, and prevents use of machine while fixing Bugs can be intermittent and hard to detect importance of self checking importance of using good programming techniques

Overconfidence Leaving out changes that are necessary (by designers) Ignoring error messages (by technicians) Not using backup devices (video or audio communication with patient)

Conclusion and Perspective Irresponsibility leads to criminal charges Responsibility leads to merit awards Importance of good software development Consequences of carelessness, cutting corners, unprofessional work, or attempts to avoid responsibility Lack of appreciation for risks Poor training Overconfidence in systems can prove deadly